1. Field of the Invention
The present invention is in the field of fail-safe actuator devices and systems incorporating the same and pertains more particularly to a fail-safe actuator for use in conjunction with a control device utilized in a hydraulic control system.
2. The Prior Art
As conducive to an understanding of the present invention, it may be noted that in certain installations and particularly in remote or unmanned installations, it is necessary that the condition of a control device such as a valve, be set to a certain sense in the event of a power failure or like happening. By way of example, in a remote automatic oil pump station, in the event a repture of the line is sensed, it is necessary that a valve be actuated to interrupt fluid flow in the oil line.
In fail-safe actuators heretofore known, and particularly in fail-safe actuators which control a valve and which utilize springs as the energy storing medium, the normal operation of the valve has also involved cycling of the spring of the actuator, i.e., illustratively when the valve is closed the tension of the spring is released and when the valve is opened by the actuator, tension is placed on the spring.
Such an actuator arrangement is shown in the U.S. Pat. No. 3,051,143 to Nee, which provides a hydraulically operated actuator having energy storage means in the form of a coil spring adapted to rotate the shaft of the actuator with drop of hydraulic pressure. Thus if said actuator is used to control a valve, upon drop in pressure to the actuator, the tensed spring will effect closure of the valve, for example. However the spring of Nee is cycled each time the actuator is energized.
As is well known, frequent cycling of a spring prematurely compromises the spring, requiring its frequent replacement to assure its effectiveness in the event of a sensed failure situation.
Also, systems as heretofore known which for normal operation require the hydraulic actuator not only shift the position of a valve but also to introduce energy into the fail-safe spring of the actuator, greater power is required than would normally be necessary to operate the valve alone, since the actuator mechanism must, in addition, compress the spring to its energy storing condition with each operating cycle of the actuator.